Life in the deep sea must adapt to unique conditions of low or no light, high pressure, low energy (except at hot vents and cold hydrocarbon seeps), and near-freezing or - at hot vents - superheated temperatures.

One unexplained but fairly common feature of deep animals is gigantism - such as oarfish, the giant squid, etc.

Amazingly, less than 1% of the deep sea has been explored. What might be left undiscovered?